(a) Field of the Invention
The present invention relates to an axial pin tumbler lock, and more particularly to an axial pin tumbler lock that cannot be easily unlocked by impressioning.
(b) Description of the Related Art
Generally, an axial pin tumbler lock, also called a tubular lock, comprises 4-7 pin tumblers located around the circumference of a circle. The body of the key is tubular and the biting corresponding to each pin tumbler is cut out of the end.
A shear line lies between a rotatable plug and a fixed shell of the lock. Each biting on the key presses one bottom pin, which in turn presses a corresponding top pin. The top surface of the bottom pin and the bottom surface of the top pin are thus moved to the shear line. When ends of all pins lie along the shear line, the plug, along with the bottom pins, is free to rotate within the shell and the lock can be unlocked. However, if one or more of the top pins projects into the plug, or one or more of the bottom pins projects into the shell, rotation of the plug is blocked by the projecting pins.
Unlike linear pin tumbler locks, in which the key is inserted from front to back, depressing one pin at a time, in a tubular lock the key presses all the pins simultaneously. This renders impressioning of the lock, that is, forcing each bottom pin precisely to the shear line, much easier than picking or impressioning its linear counterpart.
For example, it has been demonstrated that conventional tubular locks can be opened in a few seconds with a plastic tube removed from a ball-point pen. When pressure is applied, the soft plastic of the tube deforms to fit the pin tumblers, mimicking the biting of the corresponding key. The tube can then be turned as the key would and the lock opened.
Conventional tubular locks can also be easily picked by bumping; that is, inserting a tube and applying a sudden force to the end of the tube, “bumping” the pins to the shear line.
U.S. Pat. No. 4,112,820 to Conger et al. discloses a special tumbler including a modified bottom pin with a relatively smaller diameter extension protruding in the direction of the key. The key has a small diameter pin at its forward end to register with and abut the pin extension to move the bottom pin to the shear line. However, because the smaller diameter extension of the modified bottom pin lies flush with the face of the plug, this lock can be easily “bumped.”
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.